1. Field of Invention
The invention relates generally to micro-energy harvesters, and more particularly, to an improved micro-energy harvester that works with a lower input power source and works at a higher efficiency.
2. Description of Related Art
Energy harvesting, also known as power harvesting or energy scavenging, is the process by which energy is input from external input sources (e.g. solar power converting ambient energy into electrical energy, thermal energy, wind energy, salinity gradients, and kinetic energy) that are captured, built up, and stored for and delivered to small, wireless autonomous applications, like those used in wearable electronics and wireless sensor networks.
A micro-energy source has very low power, which is fed into a micro-energy harvester. Micro-energy sources include environmental low energy DC or AC power generators and anything else that can output a low DC or AC power to a micro-energy harvester. Energy harvesters are typically powered by photo cells, piezoelectric transducers or a thermoelectric generator.
A micro-energy harvester contains circuitry conversion for energy and storage from the micro-energy source. The micro-energy harvester may at times be broadly referred to as a type of energy converter. Energy converters convert radiation, kinetic and thermal energy to electrical energy, where the conversion can be to alternating current (AC) or to direct current (DC).
The electrical equivalent of the input energy that a micro-energy harvester receives is in most cases below the thresholds of electronics and would not be useful absent the micro-energy harvester. For example, a supply voltage of less than 0.7 Volt at an input of a transistor or diode is below the threshold that a transistor or diode needs to be turned on, and therefore would not be useful absent some form of transformation. A typical conventional micro-energy source is much lower (e.g., millivolts, milliamps, microamps), where power is mathematically equal to voltage multiplied by current. The constant output power that the micro-energy harvester can deliver is typically very small, but since energy can be built up in its storage element over time, an application circuit can be driven in burst mode (e.g. discontinuous mode or duty cycle mode).
A micro-energy application receives the output power built up over time by the micro-energy harvester. Because the output power is built up over time, it is important that the micro-energy application does not require “continuous” power. Thus, micro-energy applications currently do not include cell phones, laptops, or other devices requiring a battery or conventional power supplies because these applications require continuous power. The micro-energy application can be referred to as a battery free application for low-energy electronics. Similarly stated, micro-energy applications include duty cycle devices, where duty cycle devices are not always on. Duty cycle devices harvest energy for a while until they have sufficient energy, and then they execute a task. In other words, a micro-energy harvester never powers a device continuously; it is always a duty cycle.
A problem with conventional micro-energy harvesters is that they require more operating power than some micro-energy sources can provide. For example, many conventional micro-energy harvesters require more input power than a solar cell under artificial illumination can provide. Given that power is mathematically equal to voltage multiplied by current, this problem can alternatively be stated as conventional micro-energy harvesters requiring too much current and/or voltage. As a result, conventional micro-energy harvesters receive input energy from large energy converters like solar panels, thermoelectric and piezoelectric generators.
Another problem with conventional micro-energy harvesters is that they have low conversion efficiency. The main reason for this low conversion efficiency is that conventional resonant step-up converters use high turn ratios for the inductor to achieve the required output voltage. Usually self-starting circuits are on at start-up, since there is no human interaction or high operating voltage to start the circuit. Normally-on circuits draw high input current at start-up, making it impossible to use weak energy sources like small solar cells. Normally on means that the transistors of the conventional micro-energy harvester are on to begin with, that is, when the micro-energy source input is at 0 volts (and 0 volts at the transistor's gate). This problem exists because, prior to embodiments disclosed in this application, no one has discovered an improved micro-energy harvester with circuitry that can switch on when normally off (that is, turn it on when it is not already on to start with).
A further problem with conventional micro-energy harvesters is that they are not a complete solution. Conventional micro-energy harvesters lack circuitry that allows them to charge a storage element like a small rechargeable battery or super capacitor. Another problem with most conventional micro-energy harvesters is that they do not contain circuitry for their state of charge, which requires a power supervisory circuit.
A conventional micro-energy harvester is shown in U.S. Pat. Pub. No. 2010/0328972 to Pollak et al. Pollak discloses a voltage converter circuit which includes an energy storage and a switch arrangement, wherein the switch arrangement has a first switch and a second switch, where the first switch of the switch arrangement has a smaller turn-on voltage according to amount than the second switch. However, Pollak's smaller turn on voltage still requires a large input current (and thus a larger startup power), has no circuit for charging an energy storage element and no power supervisory function.